Human therapists are often quite bad as well. It took me around 12 before I found a decent one. Not saying that LLMs are better but they do theoretically have more uniform quality.
How can this even be valid scientifically
There's also a lot of credentialed professionals who got their credential decades ago and haven't at all kept up with the significant changes or new data over that time. This is a pretty big problem in all of medical care.
If a therapist found to encourage any of their patients to self-harm would lose their license to practice and would likely face prosecution. The plagiarism machine should face the same level of scrutiny.